Hey guys! I believe there are few issues with how VirtualEnvironment manages directories.
Everything works fine until you try to set HATCH_DATA_DIR=/root_project/.hatch/data - i.e. to something inside the project.

VirtualEnvironment.__init__() checks if data_directory is inside root_directory and based on that either uses "flat structure" or adds a checksum-level of directories.:

 # Conditions requiring a flat structure
elif self.root in self.data_directory.parents or [...]:
    self.storage_path = self.data_directory
    self.virtual_env_path = self.storage_path / venv_name
# Otherwise the defined app path
else:
    self.storage_path = self.data_directory / project_name / checksum
    self.virtual_env_path = self.storage_path / venv_name

But since self.data_directory is not .resolved() it can either be absolute or relative path (both being inside self.root) - based on that if produces different results: HATCH_DATA_DIR=/root_project/.hatch/data - will get a "flat structure" while HATCH_DATA_DIR=.hatch/data- will get a directory with checksum.

While [env]-build environment will always go to checksum directory no matter what.

This behavior brings two issues:

1. When HATCH_DATA_DIR=/root_project/.hatch/data regular env ends up being in "flat-structure", while -build env goes to "checksum-structure". If user runs hatch build first, it will create env under: root/.hatch/virtual/my-project/jeh852/my-project-build. Then user runs hatch run format and hatch will fail, since it will check for default env under flat structure: root/.hatch/virtual/my-project/ - dir does exist, but there is no env there => hatch will crash on the next step.
2. While using HATCH_DATA_DIR=.hatch/data (relative path) - the flat structure is completely ignored, which doesn't cause errors, but ignores scenario described here.

nit: when user sets envs.default.path= property only regular environment is created under that path. I guess -build one also should be placed there, right? It's a resource of the same Environment.

TL/DR: proposing PR to properly handle "flat structure", whenHATCH_DATA_DIR is inside root of the project.